Multilayer printed wiring board

ABSTRACT

A multilayer printed wiring board wherein circuits are provided respectively on both the faces of a substrate and subsequent circuits are provided in order and via insulating layers respectively on the outsides of the preceding circuits, characterized in that the insulating layers comprise an epoxy resin, polyvinyl acetal resin, melamine or urethane resin and rubber-modified epoxy resin in a specific mixing ratio.

This is a divisional of application Ser. No. 08/516,480 filed on Aug.17, 1995, now U.S. Pat. No. 5,718,039

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a multilayer printed wiring board for use inmaking electronic circuits and a process for manufacturing said wiringboard.

2. Prior Art

A printed wiring board for use in making electronic circuits hasheretofore been manufactured by impregnating a glass cloth with athermosetting resin such as an epoxy resin and then making the thusimpregnated resin semi-cured to obtain a prepreg, placing one copperfoil facing to one side of the thus obtained prepreg and another copperfoil facing to the other side thereof, pressing the. whole togetherunder pressure and heat by the use of a press to prepare a copperfoil-applied laminate, and then subjecting to thus prepared laminate tocircuit formation by etching and to perforation.

In this case, there is used a copper foil which has been subjected toso-called unevenness or roughnss formation at one face of the copperfoil in order to ensure secure adhesion of the copper foil to theprepreg.

The above copper foil-applied laminate is etched so as to form circuitsrespectively on both the sides of the laminate thereby to fabricate aninner-layer member. Copper foils are further adhered via the prepregsonto both the sides of the inner-layer member, respectively, and thenetched to form outer-layer circuits whereby a multilayer printed wiringboard is also manufactured as illustrated in FIG. 1 which is a schematiccross-sectional view of a multilayer printed wiring board having twoinner circuits and two outer circuits.

In this case, the inner layer circuits (copper foils) formed in theinner layer member will not fully be adhered to the prepregs of theouter layer member since the exposed surfaces of the inner circuits aresmooth surfaces (which are called "glossy face"). Thus, it is a commonpractice that the surface of the circuits (copper foils) of the innerlayer member is subjected to black oxide treatment to ensure the secureadhesion of the inner layer member to the outer layer member and then amultilayer board is fabricated. The adhesiveness of the surface of theinner-layer circuits (copper foils) formed in the inner-layer memberrespectively to the prepregs of the outer-layer member in greatlyenhanced by the black oxide treatment and, thus, the heat resistance andmoisture resistance of the resulting multilayer printed wiring boardwill amount to a practically satisfiable level. The multilayer printedwiring boards produced by the above method have been used in variousfields such as electrical and electronic fields.

Although the adhesion of the inner-layer circuits (copper foils) formedin the inner-layer member respectively to the prepregs of theouter-layer member is improved, various problems are raised in otheraspects. According to "Technical Manual of Printed Circuits", p. 261,published by Nikkan Kogyo Newspaper Publishing Co.", the variousproblems include the oxide film is dissolved in hydrochloric acid or thelike whereby haloing phenomena are presented and the insulationproperties and inter-layer connection reliability are apt to bedecreased. The problems also include difficulties in grinding orpolishing thin inner-layer circuits (copper foils) and operationaldifficulties due to the use of a concentrated alkaline solution or thelike for the black oxide treatment.

To solve these problems, another method such as one described in, forexample, Japanese Patent Gazette No. Hei 4-19306 (or No. 19306/92) hasbeen proposed as a substitute for the black oxide treatment; however,this substitute dose not come to be widely used since it needs specificchemicals and the like. As a method which dispenses with black oxidetreatment, there is also known a method using so-called double-treatcopper foils both the faces of which have been subjected to rougheningtreatment. The double-treat copper foils are not generally used sincethey need a complicated process for being produced, incur a higher costand raise problem such as insufficient peeling-off of the photoresisttherefrom at the time of formation of circuits due to close adhesion ofa photoresist to the copper foils.

SUMMARY OF THE INVENTION

The object of this invention is to solve the problems involved in theabove black oxide treatment and also to provide a multilayer printedwiring board having high electrical properties, heat resistance and thelike as well as a process for producing said wiring board.

The above object may be achieved by providing an insulation layer havinga specific composition without subjecting the copper foils (inner-layercircuits) to black oxide treatment.

This invention resides in a multilayer printed wiring board prepared byproviding both the faces of a substrate (prepreg for example)respectively with circuits, providing via insulating layers on theoutsides of said circuits respectively with circuits and then repeatingsuch provision on the outsides as above, characterized in that saidinsulating layers comprise 40-70 wt. % of an epoxy resin, 20-50 wt. % ofa polyvinyl acetal resin and 0.1-20 wt. % of a melamine resin orurethane resin, each based on the total amount of the resin components,with the proviso that 50-80 wt. % of said epoxy resin is arubber-modified epoxy resin.

The multilayer printed wiring board of this invention is characterizedby using the insulating layers without subjecting the circuits (copperfoils) provided respectively on both the faces of the substrate to blackoxide treatment.

The epoxy resin used in the above insulating layer may be such that itis a commercially available one for use in laminates and moldingelectronic elements. These commercially available resins may be usedwithout any specific limitation. The epoxy resins so used include abisphenol A-type epoxy resin, bisphenol F-type resin, novolak-type epoxyresin, o-cresol novolak-type epoxy resin, glycidyl amine compounds suchas triglycidyl isocyanurate and N, N-diglycidyl aniline, and brominatedepoxy resins such as tetrabromo-bisphenol A diglycidyl ether. Theseepoxy resins may be used singly or jointly. Further, they are notparticularly limited in polymerization degree and epoxy equivalent asepoxy resins.

The curing agents for use in the epoxy resins preferably includegenerally known latent curing agents such as dicyanodiamide, organichydrozide and imidazole, and a phenol novolak resin which is difficultlycurable at normal temperature.

The optimum amounts of these curing agents added to the respective epoxyresins are known and, however, the amounts added may be varied as far asthe curing agents maintain their effects as curing agents. Further,these curing agents may be used singly or jointly. Epoxy resin cureaccelerators such as a tertiary amine may preferably be additionallyused.

The amount of the epoxy resin used in said insulating layer is 40-70% byweight of the total of the resin ingredients. If the amount used is lessthen 40 wt. % then the resulting printed wiring board will bedeteriorated in electrical properties and heat resistance, while if theamount used exceeds 70 wt. % then the resin will run too much from theinsulating layer by using a press at the time of the formation of amultilayer whereby adhesiveness between the circuits and insulatinglayers and insulating property thereof are lowered.

The rubber-modified epoxy resins used as a part of the epoxy resinswhich constitute the insulating layer in this invention may becommercially available ones for use as adhesiveness or paints, and thesecommercially available ones may be used without any particularlimitations. They are illustrated by "EPICION TSR-960" (tradename andproduced by Dai Nippon Ink Mfg. Co.), "EPOTOHTO YR-102" (tradename andproduced by Tohto Kasei Co.), "SUMIEPOXY ESC-500" (tradename andproduced by Sumitomo Chemical Co.) and "EPOMIK VSR 3531" (tradename andproduced by Mitsui Petrochemicals Co.). These rubber-modified epoxyresins may be used singly or jointly. The amount of the rubber-modifiedepoxy resins used herein is 5-80% by weight of the total of the epoxyresins.. The adhesion of the insulating layer to the surface of thecircuits (copper foils) which are not subjected to black oxide treatmentis remarkably improved in adhesiveness by the use of the rubber-modifiedepoxy resin in the insulating layer. If, however, the amount used isless than 5 wt. % then an improvement in adhesiveness will not be made,while the amount used is 80 wt. % then the insulating layer will belowered in heat resistance.

The polyvinyl acetal resin used in the insulating layer according tothis invention may be a resin which is synthesized by reacting apolyvinyl alcohol with an aldehyde. At the present, reaction productsobtained by the reaction of polyvinyl alcohols having variouspolymerization degrees with at least one or more kinds of aldehydes, aremarketed as polyvinyl acetal resins for use in paints and adhesives and,however, the polyvinyl acetal resin may be used in this inventionirrespective particularly of the kinds of aldehydes and degree ofacetalization. Although the degree of polyacriozation of polyvinylalcohol as a starting material is not particularly limited, it ispreferably in the range of 2000-3500 in view of the solubility tosolvents and heat resistance of the resulting insulating layer. Further,modified polyvinyl acetals into the molecular of which carboxyl group orthe like has been introduced are also commercially available and theymay be used without any particular limitation if they raise no problemsas to compatibility with epoxy resins. The amount of the polyvinylacetal resins used is 20-50% by weight of the total of the resiningredients. If the amount use is less than 20 wt. % then theaforementioned improvements in the run of the epoxy resin from theinsulating layer are not made, while if the amount used exceeds 50 wt. %then the resulting cured insulating layer will increase in waterabsorptivity whereby the printed wiring board is deteriorated inwaterproofing as such.

The adhesive used in this invention is, in addition to the aboveingredient, incorporated with a melamine resin or a urethane resin as acrosslinking agent for the above polyvinyl acetal resin.

The melamine resin used herein may be a commercially availablealkyl-etherified (alkylated) melanime resin for use for paints. Such acommercially available melamine is illustrated by a methylated melamineresin, n-butylated melamine resin, iso-butylated melamine resin or mixedalkylated melamine resin. The molecular weight and the alkylation degreeof these melamine resins are not particularly restricted.

The urethane resins used in this invention include resins which containisocyanate groups in the molecule and are commercially available for usefor adhesives and paints. They are illustrated by reaction products ofpolyisocyanate compounds such as tolylene diisocyanate, diphenylmethanediisocyanate and polymethylenepolyphenyl polyisocyanate, with polyolssuch as trimethylalpropane, polyetherpolyol and polyesterpolyol. Thesereaction products (resins) are very reactive as resins and may bepolymerized in the presence of atmospheric moisture in some cases and,therefore, it is preferable that these resins be stabilized with aphenol or oxime to prepare urethane resins called block isocyanates, foruse in this invention.

The amount of a melamine resin or urethan resin incorporated in theinsulating layer of this invention is 0.1-20% by weight of the total ofthe resin ingredients. If the amount used is less than 0.1 wt. % thenthe polyvinyl acetal resin will not fully be crosslinked thereby theresulting insulating layer is deteriorated in heat resistance, while ifthe amount used is more than 20 wt. % then adhesiveness of the resultinginsulating layer to copper foils having not been subjected to surfaceroughening will be lowered.

In the preparation of the insulating layers, there may be used asrequired inorganic fillers. typified by talc and aluminum hydroxide aswell as an anti-foaming agent, a levelling agent, a coupling agent andthe like, in addition to the aforementioned essential ingredients. Theabove optional ingredients are effective to improve the resultinginsulating layer in levelness and smoothness, enhance thefire-retardation of the layer and lower a cost for the preparation ofthe layer.

A process for a multilayer printed wiring board of this invention willbe explained hereunder.

This invention resides in a multilayer printed wiring board and aprocess for the preparation thereof which comprises:

(1) a step of adhering copper foils respectively to both the faces of asubstrate, masking the copper foils at predetermined portions of thenon-adhesive faces thereof and then etching the partly masked copperfoils to form circuits thereon,

(2) a step of applying copper foils having a semi-cured insulating layerformed on one face of the respective copper foils, the insulating layercomprising 40-70 wt. % of an epoxy resin and a curing agent therefor and20-50 wt. % of a polyvinyl acetal resin and 0.1-20 wt. % of a melamineresin or urethane resin, each based on the total of the resiningredients with the proviso that 5-80% by weight of said epoxy resin isa rubber-modified epoxy resin, facing the insulating layer respectivelyto said circuits formed on both the faces of the substrate, and thenpressing the whole together under heat thereby to cure the insulatinglayers and simultaneously laminate the insulating layers respectivelywith said circuits formed on both the faces of the substrate and

(3) a step of masking the predetermined portions of the copper foilsrespectively laminated via the insulating layers with said circuits andthen etching the partly masked copper foils to form circuitsrespectively on the insulating layers.

Further, according to this invention, the above-mentioned steps (2) and(3) may be repeated as many times as desired to increase layers.

An ordinary method comprises the steps of adhering copper foilsrespectively to both the faces of a substrate such as anepoxy-impregnated glass cloth prepreg, masking predetermined portions ofthe non-adhesive face (which is not adhered to the substrate) of copperfoils with a photoresist or the like and then etching the partly maskedcopper foils to obtain predetermined circuits. The circuits (copperfoils) so obtained are hitherto subjected to black oxide treatment and,however, such black oxide treatment is not carried out in thisinvention.

In general, inner-layer circuits are freed from foreign matters locatedon the surface of the circuits and attachments formed thereon at thetime of pressing by use of the steps of washing and degreasing, andthereafter subjected to black oxide treatment. In this invention, awashing step and a degreasing step are basically not necessary and,however, these steps may be carried out if necessary depending on thestate of the inner-layer circuits. Further, the material for theinner-layer circuit board and the thickness of copper foils of theinner-layer circuits are not particularly limited as far as said circuitboard and copper foils are such that they are now used in a multilayerprinted wiring board.

According to this invention, copper foils having a semi-cured insulatinglayer on one face thereof are applied respectively to circuits (copperfoils) formed on both the faces of a substrate with the semi-curedinsulating layer facing to the substrate, the semi-cured insulatinglayer comprising 40-70 wt. % of an epoxy resin and a curing agenttherefor, 20-50 wt. % of a polyvinyl acetal resin and 0.1-20 wt. % of amelamine or urethane resin, each based on the total of the resiningredients, with the proviso that 5-80 wt. % of the epoxy resin is arubber-modified epoxy resin, the whole is then pressed together underheat thereby to cure the semi-cured insulating layers and simultaneouslylaminate the insulating layers respectively with the circuits formed onboth the faces of the substrate.

The copper foil having a semi-cured insulating layer on one face thereof(semi-cured insulating layer-applied copper foil) is prepared by coatinga copper foil on one face thereof with a resin varnish obtained bydissolving the aforementioned resin ingredients in an industriallyinexpensive general-purpose solvent such as methyl ethyl ketone ortoluene, drying the coated resin varnish and then semi-curing (partlycuring) the dried varnish.

The copper foils used herein include a rolled copper foil and anelectrolytic copper foil and have a thickness of preferably 9-100 μm,more preferably 12-35μm. The surface treatment and rust preventivetreatment of the copper foil are not particularly restricted. There areno particular limits to the thickness of the insulating layer and,however, it is desirable that the insulating layer be at least 50μm inthickness after semi-cured in view of necessity to ensure insulationresistance between layers, loss in thickness due to pressing, and thelike. There are set no particular limits to the thickness of theinsulating layer. If, however, the insulating layer is made thicker thanis required, then the resin may run from the end of the insulating layerat the time of pressing. Thus, it is preferable that the insulatinglayer be about 500μm or less in thickness.

According to this invention, a multilayer printed wiring board can beproduced without subjecting the circuits formed on the inner-layermember to black oxide treatment by using insulating layers having aspecific composition.

While the multilayer printed wiring board of this invention maintainselectrical properties and heat resistance equal to those of aconventional one, various problems as to the black oxide treatment canbe solved. The conditions for lamination at this invention do notrequire any change of the conventional conditions. This invention isalso advantageous in that the unevenness and migration on the surface ofa multilayer printed wiring board are avoided since fibers such as glasscloth are not used in insulating layers between the inner-layer circuitsand other circuits such as the outer-layer circuits. A process forfabricating a multilayer printed wiring board by the use of insulatinglayer-applied copper foils in which the copper foil and the insulatinglayer are integrated, is illustrated in "Printed Circuit WorldConvention VI" T9. The process of this invention, however, is differentfrom the above illustrated process in that the insulating layeraccording to this invention is uniform and tack-free at room temperatureand in the form of being semi-cured without need of a protective filmthereby to dispense with a step of peeling the protective film, thismaking the process according to this invention industrially moreadvantageous.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view of an embodiment of aconventional multilayer printed wiring board having glass cloth prepregsubstrate therein or a novel one of this invention having insulatinglayers therein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention will be better understood by the following Examples andComparative Examples wherein epoxy resins which are not modified withrubber are simply called "epoxy resins", while epoxy resins which havebeen modified with rubber are called "rubber-modified epoxy resins".

EXAMPLE 1

(1) Preparation of an inner-layer member

35μm-thick electrolytic foils one face of each of which has beensubjected to roughening treatment were applied respectively to both thefaces of a substrate consisting of 8 sheets of a commercially available0.1 mm-thick epoxy resin-impregnated glass cloth prepreg, the 8 sheetsbeing placed one upon another, with the roughened face of the copperfoil facing to the substrate as shown in FIG. 1, after which the wholewas pressed together at a pressure of 30 kgf/cm² and a temperature of170° C. for 60 minutes to prepare an inner-layer member wherein thecopper foil has been applied to each face of the substrate thereby toform a laminate. Both the faces (copper foils) of the thus formedlaminate were masked at their predetermined portions and then etched byan ordinary method to form circuits.

(2) Preparation of insulating layer-applied copper foils

In a 1:1 toluene/methanol mixed solvent were dissolved 40 parts byweight of an epoxy resin (tradename, EPOMIC R-301, produced by MitsuiPetrochemicals Co.), 20 parts by weight of a rubber modified epoxy resin(tradename, EPOTOHTO YR-102, produced by Tohto Kasei Co.), 30 parts byweight of a polyvinyl acetal resin (tradename, DENKA BUTYRAL No. 5000A,produced by Denki Kagaku Kogyo Co.), 10 parts by weight as a solid of amelamine resin (tradename, YUBAN 20 SB, produced by Mitsui Toatsu KagakuCo.), 2 parts by weight of a latent epoxy resin curing agent(dicyandiamide, reagent) which was added in the form of adimethylformamide solution of 25 wt. % of a solid, and 0.5 parts byweight of a cure accelerating agent (tradename, CURE SOL 2E4MZ, producedby Shikoku Kasei Co.), thereby to prepare a resin varnish containing 25wt. % as solids.

The thus prepared resin varnish was coated on the roughened face of35μm-thick electrolytic copper foils, air-dried and then heated at 150°C. for 7 minutes thereby to obtain semi-cured insulating layer-appliedcopper foils. The insulating layers so obtained at this point was each100μm in thickness.

(3) Preparation of a multiple printed wiring board

Both the faces of the inner-layer member prepared at the above step (1)were washed with purified water, the insulating layer-applied copperfoils prepared at the above step (2) were placed respectively on boththe washed faces with the insulating layer side of the insulatinglayer-applied copper foil facing to said washed face, thereafter thewhole was pressed together at 30 kg/cm² and 170° C. for 60 minutes toform outer-layer members respectively on both the faces of theinner-layer member, masking predetermined portions of both the faces(copper foils) of the outer-layer member formed at the above step (3),and then the masked faces (copper foils) of the outer-layer member wereetched by an ordinary method to form outer-layer circuits, thereby toprepare a multilayer (4 layers in this case) printed wiring board asshown in FIG. 1.

EXAMPLE 2

The procedure of Example 1 was followed except that 20 parts by weightof a rubber-modified epoxy resin (tradename, SUMIEPOXY ESC-500, producedby Sumitomo Kagaku Co.) were substituted for 20 parts by weight of therubber-modified epoxy resin (tradename, EPOTOHTO YR-102, produced byTohto Kasei Co.) used at the above step (2), thereby to prepare amultilayer (4-layer) printed wiring board.

EXAMPLE 3

The procedure of Example 1 was followed except that a urethane resin(tradename, COLONATE AP-Stable, produced by Nippon Polyurethane Co.) wassubstituted in the same amount as a solid (10 parts by weight) for themelamine resin (tradename, YUBAN, produced by Mitsui Toatsu Kagaku Co.),thereby to prepare a multilayer (4-layer) printed wiring board.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was followed except that 35μm-thick,surface-roughened electrolytic copper foils were substituted for theinsulating layer-applied copper foils used in Example 1 and commercialavailable 0.1 mm-thick epoxy resin-impregnated glass cloth prepregs wereused as insulating layers, thereby to prepare a multilayer (4-layer)printed wiring board.

COMPARATIVE EXAMPLE 2

The procedure of Example 1 was followed except that the same copperfoils and prepregs as used in Comparative Example were used, and, beforethe multilayer formation of an inner-layer member used, the surface ofthe inner-layer circuits (copper foils) was treated with a solutioncontaining 31 g/l of sodium hydrochlorite, 15 g/l of sodium hydroxideand 12 g/l of trisodium phosphate, at 85° C. under agitation for 3minutes thereby to effect a black oxide treatment, thus preparing amultilayer (4-layer) printed wiring board.

COMPARATIVE EXAMPLE 3

The procedure of Example 1 was followed except there was used a resinvarnish prepared by substituting an epoxy resin (tradename, EPOTOHTOYD-128, produced by Tohto Kasei Co.) for the rubber-modified epoxy resin(tradename, EPOTOHTO YR-102, produced by Tohto Kasei Co.) used as oneingredient of the insulating layer prepared at the aforementioned step(2) of Example 1, thereby to prepare a multilayer (4-layer) printedwiring board.

COMPARATIVE EXAMPLE 4

The procedure of Example 1 was followed except that there was used aresin varnish having a solid content of 48 wt. % and containing 70 partsby weight of an epoxy resin (tradename, EPOMIC R-301, produced by MitsuiPetrochemicals Co.), 20 parts by weight of a rubber-modified epoxy resin(tradename, EPOTOHTO YR-102, produced by Tohto Kasei Co.) and 10 partsby weight of a polyvinyl acetal resin (tradename, DENKA BUTYRAL No.5000A, produced by Denki Kagaku Kogyo Co.), the above resins being thesame as those used as the ingredients of the insulating layer preparedat the step (2) of Example 1, thereby to prepare a multilayer (4-layer)printed wiring board.

The multilayer printed wiring boards (the laminates) obtained inExamples 1-3 and Comparative Examples 1-4 were evaluated for thefollowing performances and properties. The results are as shown in Table1.

1. Normal-state peeling strength between the copper foil on the surfaceof the inner-layer member and the insulating layer.

In accordance with JIS C 6481

2. Solder heat resistance

In accordance with JIS C 6481

3. Surface resistance

In accordance with JIS C 6481

4. Haloing resistance

The boards are each perforated to make a 0.4 mm φ through hole, and theperforated boards are each immersed in a 1:1 aqueous solution ofhydrochloric acid at room temperature to visually determine whetherhaving occurs or not.

5. Moisture proofing

The boards are boiled in purified water for 2 hours and then immersed ina solder bath at 260° C. for 30 seconds to visually determine whetherswelling occurs or not.

6. Thickness of insulating layer before or after pressed

Thickness found

                                      TABLE 1    __________________________________________________________________________                                     Thickness of          Peeling               Solder heat                      Surface                           Haloing                                Moisture-                                     insulating layer    Ex. and          strength               resistance                      resistance                           resistance                                proofing                                     before or after    Comp. Ex.          (Kgf/cm)               (sec)  (Ω)                           *1   *2   pressed (μm)    __________________________________________________________________________    Ex. 1 1.31 more than 120                      6 × 10.sup.14                           ∘                                ∘                                     100/95    Ex. 2 1.28 more than 120                      5 × 10.sup.13                           ∘                                ∘                                     100/90    Ex. 3 1.55 more than 120                      7 × 10.sup.15                           ∘                                ∘                                     100/95    Comp. Ex. 1          0.26 60     1 × 10.sup.15                           ∘                                x    100/90    Comp. Ex. 2          1.35 more than 120                      1 × 10.sup.15                           x    ∘                                     100/90    Comp. Ex. 3          0.72 more than 120                      9 × 10.sup.14                           ∘                                ∘                                     100/70    Comp. Ex. 4          0.62 more than 120                      8 × 10.sup.14                           ∘                                ∘                                     100/20    __________________________________________________________________________     *1; ∘: No haloing x: Haloing occurred     *2; ∘: No swelling x: Swelling occurred

It is apparent from Table 1 that the multilayer printed wiring boardexhibits performances and properties equal to those of a conventionalone even if the former is not subjected to black oxide treatment, and,further, problems as to the black oxide treatment are avoided.

This invention enables the black oxide treatment to be dispensed with inthe preparation of a multilayer printed wiring board and also enablesvarious problems raised by the black oxide treatment to be eliminated.In addition, the multilayer printed wiring board of this invention iscomparable to a conventional one in performances and will not raise anyproblems as to surface smoothness and migration. Further, the insulatinglayer used in this invention is uniform and does not need a protectivefilm. Thus, in a case where, the insulating layer is used in thepreparation of an insulating layer-applied copper foil, thelayer-applied copper foil so prepared will be satisfactorily easilyhandled and operated.

What is claimed is:
 1. A multilayer printed wiring board whereincircuits are provided respectively on both faces of a substrate andsubsequent circuits are provided in order and via insulating layersrespectively on the outsides of the preceding circuits, comprising:saidinsulating layers comprise 40-70% by weight of epoxy resin, 20-50% byweight of polyvinyl acetal resin and 0.1-20% by weight of melamine resinor urethane resin, each based on a total of the resin ingredients, withthe proviso that 50-80% by weight of said epoxy resin is arubber-modified epoxy resin.
 2. A multilayer printed circuit board,comprising:a substrate; a first circuit adhered to a face of saidsubstrate, said first circuit being formed of a copper foil; at least afirst and second insulated circuit layer, each said insulated circuitlayer including an insulation layer adhered to a copper circuit; saidcopper circuit being formed of said copper foil; said insulation layerof said first insulated circuit layer contacting said first circuit;said insulation layer of said second insulated circuit layer contactingsaid copper circuit of said first insulated circuit layer; saidinsulation layer comprising:40-70% by weight epoxy resin with 50-80% ofsaid epoxy resin being a rubber modified epoxy resin, 20-50% by weightpolyvinyl acetal resin, and 0.1-20% by weight of one of melamine resinand urethane resin.
 3. The multilayer printed circuit board of claim 2,wherein said epoxy resin includes at least one selected from the groupconsisting of a bisphenol A-type epoxy resin, a bisphenol F-type epoxyresin, a Novolak-type epoxy resin, a glycidyl amine compound, and abrominated epoxy resin;said glycidyl amine compound being at least oneselected from the group including a triglycidyl isocyanurate and a N,N-diglycidyl aniline; said brominated epoxy resin being at least oneselected from the group including tetrabromo-bisphenol A diglycidylether.
 4. The multilayer printed circuit board of claim 2, wherein acuring agent for said epoxy resin is a latent curing agent.
 5. Themultilayer printed circuit board of claim 4, wherein said latent curingagent includes at least one selected from the group consisting of adicyandiamide, an organic hydrozide, an imidazole, and a phenol novolakresin.
 6. The multilayer printed circuit board of claim 2, furthercomprising:an epoxy resin cure accelerator; and said epoxy resin cureaccelerator is selected from a group including an epoxy resin cureaccelerator having a tertiary amine.
 7. multilayer printed circuit boardof claim 2, wherein:said polyvinyl acetal resin used in said insulationlayer is a resin which is synthesized by reacting a polyvinyl alcoholwith at least one aldehyde; said polyvinyl alcohol has a degree ofpolyacriozation in the range of 2000-3500; said polyvinyl acetal resinis modified with a carboxyl group to produce a modified polyvinyl acetalresin; and said modified polyvinyl acetal resin is compatible with saidepoxy resin.
 8. The multilayer printed circuit board of claim 2, whereinsaid melamine resin includes at least one selected from the groupconsisting of an alkyl-etherified (alkylated) melamine resin, amethylated melamine resin, an n-butylated melamine resin, aniso-butylated melamine resin, and a mixed alkylated melamine resin. 9.The multilayer printed circuit board of claim 2, wherein:said urethaneresin is produced from reaction products of a polyisocyanate compoundcombined with a polyol; said polyisocyanate compound includes at leastone selected from the group consisting of a tolylene diisocyanate, adiphelymethane diisocyanate, and a polymethylenepolyphenylpolyisocyanate; said polyol includes at least one selected from thegroup consisting of a trimethylalpropane, a polyetherpopyol, and apolyesterpolyol.
 10. The multilayer printed circuit board of claim 2,wherein said insulation layer further comprises at least one selectedfrom the group consisting of an inorganic filler, an anti-foaming agent,a leveling agent, and a coupling agent; andsaid inorganic filler beingat least one selected from a group including a talc, and an aluminumhydroxide.
 11. A multilayer printed circuit board, comprising:asubstrate; a first circuit adhered to a face of said substrate, saidfirst circuit being formed of a copper foil; at least a first and secondinsulated circuit layer, each said insulated circuit layer including aninsulation layer adhered to a copper circuit; said copper circuit beingformed of said copper foil; said insulation layer of said firstinsulated circuit layer contacting said first circuit; said insulationlayer of said second insulated circuit layer contacting said coppercircuit of said first insulated circuit layer; said insulation layercomprising:40-70% by weight epoxy resin with 5-80% of said epoxy resinbeing a rubber modified epoxy resin; 20-50% by weight polyvinyl acetalresin; and 0.1-20% by weight of one of melamine resin and urethaneresin.